Physics of Bicycles: Question #1.
 

How come it's nearly impossible to balance a bicycle standing still, but it's easy as pie to balance a bicycle with any sort of front or back movement?

Because the forward/back movement corrects the balance?

When you are moving, and you start to lean, you start to turn. You have time to correct yourself.

When you are still, and you start to lean, you fall over, unless you very quickly shift your weight.

When you move you activate the pixie dust which keeps you from falling over


http://pixyland.org/peterpan/Imagezz...2_adj_680h.jpg

OMG when your tire wheel tur they act as gyroscopes.

sit on a bar stool with the bie rear tire in the and your hands on the pedels....
spn the pedels and lean the wheel left or right the bar stool will swivel in that direction.

When you are moving, minor adjustments in the steering make very rapid sideways adjustments of the wheels, making it easy to keep the wheels under you. If you try to ride very slowly, you'll wind up making exagerated steering motions trying to accomplish the same thing.

Don't think you get much gyro action at < 1mph....

Momentum. Mass*velocity give you momentum, which is the reason the wheel resists changes to movement. Thus, once you get rolling, the bike is hard to tip over. Heavier wheels have more momentum, which is why they're easy to keep spinning once they get up to speeds. It's also why light wheels accelerate faster -- because there is less momentum.

Experiment: take off the front wheel and hold it by the skewer in both hands. Spin the wheel really fast and try to move the wheel from vertical to horizontal (90 degrees). It's that "force."

And if we're talking about track standing, it's because moving the wheels on a 20lb bike adjusts your center of gravity on the bike faster than moving your 160lb (or whatever) body.

And to the more technical, by center of gravity i mean the lack of a moment (the thing that makes you tip over) about the wheel contact patch.

+1 here are some nice demonstrations:
http://www.youtube.com/watch?v=545GwnupKAE
http://www.youtube.com/watch?v=u47eMRgJoBQ

Gyroscopic forces and simple resistance to tilting doesn't explain it: http://www.losethetrainingwheels.org...px?Lev=2&ID=34

Also read the wikipedia link posted. It's not a simple answer.

My single speed bike is an old road bike with a non-stock fork. It's impossible to ride no-handed for any distance. I didn't know why this was until I noticed that it has nearly zero trail. With even a few fingers on the bars, it's fine. I think I keep steering it underneath me without consciously noticing.

Track standing works more like a unicycle. It's a dynamic system of balance; you can't track stand with your brakes locked. You fall each way for a short period of time.

Why can't a bicycle stand up by itself?



Because it's too tired.

I just read both the wiki and the later posted article and I still have no idea how a bicycle stay upright.

Maybe I'll learn once I get into the meat of my Engineering classes (if I decide to go the mechanical route).

Um, no. Gyroscopic forces have almost nothing to do with actually riding a bike.

This is absolutely correct.

Replace both wheels of a bicycle with small skies and ride down a ski slope. No gyro action, and bike will stay upright.

See it is the pixie dust.:)

For a starting point, assume the bike is balanced and moving forward. The center of gravity is over the tire contact patches. If it starts to tilt, the front wheel turns in the direction of the tilt, and the bike moves in that direction to place the tire contact patches under the center of gravity again. This happens by itself at speed, due to the geometry of the fork (why you can ride no-hands): and by the rider's hand movements at low speeds.

As in track standing, you rock the bike back and forth, continually correcting your balance to stay upright.

+1 it isnt so much that balance is easier with movement, but that movement allows you to correct balance with less effort.

I like this statement!

I think of it as, bikes stay upright when on the average the bike center of gravity remains above the line that connects the wheel contact patches. Because it's an inherently unstable situation, it takes control to maintain this balance.

There's also a self-centering force due to front wheel trail that tends to reduce the control effort needed, at least at speeds above a threshold.

Sitting at a bar when you wrote this? Or after the bar? :)

That makes sense as long as you're going straight. So what happens when you lean over in a turn?

What matters is the resultant of the force vectors for gravity and cornering force. The resultant vector has to intersect the line between the contact patches. If you were cornering at 1g, the bike would be leaned over at 45°, and the two vectors (cornering force horizontal, gravity vertical) would be equal in magnitude, and their combination would be at 45°. The same sort of thing would be true at lesser cornering forces, with the angle being different, of course.

the inside of the corner puts you closer to the pixie dust, hence the effect is stronger in preventing you from falling, but the magic in the pixie dust gets used up quicker so you have to keep steering into the pixie dust, thats why you lean into corners, not out of them.